Methadone safety: conversion, QTc and interactions

Methadone is effective but unforgiving: its potency relative to morphine rises with prior dose, it accumulates for days, and it prolongs QTc; convert conservatively, titrate slowly, screen ECGs and check every interacting drug.

1. Conversion is nonlinear. Methadone's potency relative to morphine increases as the prior opioid dose rises, so fixed-ratio equianalgesic conversion overestimates the safe dose and can cause fatal respiratory depression ¹².
2. Underestimate rather than overestimate. Because of high inter-patient variability in absorption, metabolism and relative potency, the label says it is safer to underestimate the 24-hour dose and provide rescue medication ².
3. It accumulates. Elimination half-life is 8 to 59 hours and potency increases with repeated dosing through accumulation; steady state and full effect take at least 3 to 5 days, so dose increases should be no more frequent than every 3 to 5 days ².
4. Respiratory depression lags analgesia. The peak respiratory depressant effect occurs later and lasts longer than the peak analgesic effect, so monitor closely after starting or increasing ².
5. Screen the QTc. The expert panel advises an ECG before treatment, within 30 days and annually, with more if the dose exceeds 100 mg/day or there is unexplained syncope or seizures ³.
6. Act on QTc thresholds. For 450–500 ms, discuss risks and monitor more often; above 500 ms, consider stopping or reducing methadone, removing contributors such as hypokalemia-promoting drugs, or switching therapy ³.

Starting a CYP3A4, 2B6, 2C19, 2C9 or 2D6 inhibitor, or stopping a CYP3A4, 2B6, 2C19 or 2C9 inducer, can produce a fatal methadone overdose; review every medication change ².

• Methadone carries a boxed warning for QT prolongation and torsades de pointes ².
• APS–CPDD, with the Heart Rhythm Society, favors alternative opioids for patients at high risk of QTc-related complications ¹.
• Inform patients of arrhythmia risk and ask about structural heart disease, arrhythmia and syncope ³.
• Methadone-associated overdose deaths rose in parallel with increased methadone use for chronic pain ¹.
• For opioid use disorder induction, the first dose is 20–30 mg (maximum 30 mg), and day-one total should not ordinarily exceed 40 mg ².
• For same-day induction adjustments, wait 2–4 hours for peak levels before giving an extra 5–10 mg ².
• Most APS–CPDD recommendations rest on low-quality evidence; none on high-quality evidence ¹.

• Converting high-dose morphine to methadone with a fixed ratio ¹.
• Increasing methadone every day for uncontrolled pain ².
• Adding a strong CYP3A4 inhibitor without reassessing methadone dosing ².
• Stopping methadone abruptly in a physically dependent patient; rapid reduction has caused serious withdrawal ².